The present invention relates to a Coriolis mass flowmeter and, more particularly, to a loop-type Coriolis mass flowmeter in which two parallel bent tubes are connected in series.
When a tube through which a fluid to be measured flows is supported at the one or both ends thereof and vibrated at this supporting point in a direction vertical to the direction of fluid flow in the tube, Coriolis forces acting on the tube (a tube to which vibrations are applied is hereinafter referred to as a flow tube) are proportional to a mass flow rate. A mass flowmeter based on this principle (a Coriolis mass flowmeter) is well known. Flow tubes used in this Coriolis mass flowmeter are broadly classified into bent tubes and straight tubes according to their shape.
In a Coriolis mass flowmeter of the straight tube type, a mass flow rate is measured as a difference in the displacement of the straight tube caused by the Coriolis forces between the supporting point and the middle portion of the straight tube, i.e., a phase difference signal, when the Coriolis mass flowmeter is vibrated in a direction vertical to the axis of the straight tube supported at both ends. In such Coriolis mass flow meters of the straight tube type, it is difficult to achieve high detection sensitivity though they are simple, compact and robust in construction.
In contrast, Coriolis mass flowmeters of the bent tube type, in which the shape of the bent tube can be selected appropriately to effectively take out Coriolis forces, can detect ass flow rates with high sensitivity. It is also known that in order to more efficiently drive this measuring bent tube, the bent tube in which a fluid to be measured flows is constituted of two parallel tubes.
FIG. 4 schematically shows this conventional Coriolis mass flowmeter of the two parallel bent tube type. As shown in the figure, the flow tube is fabricated from two parallel U-shaped tubes in which a branching portion is formed on the inlet side of the fluid to be measured and a junction portion is formed on the outlet side thereof. The fluid to be measured is equally divided into two flow tubes on the inlet side and joins on the outlet side of the flow tubes. By causing the fluid to be measured to flow equally in two flows in this manner, it is possible to constantly make the natural frequency of two flow tubes equal even when the kind of fluid changes, or the temperature fluctuates. It is known that this permits efficient and stable driving of flow tubes and makes it possible to form a Coriolis mass flowmeter that is not affected by external vibrations or temperature changes.
Pressure losses or clogging of flow tubes with the fluid to be measured may sometimes occur in the branching portion at the inlet of the fluid and the junction portion at the outlet thereof. This poses a problem especially in the case of high-viscosity fluids and liquids such as perishable and easy-to-clog foods. Furthermore, when one flow tube is recovered from clogging during the flushing of flow tubes with a branching portion, the flushing performance of the other flow tube deteriorates, leading to a long flushing time.
A loop-type Coriolis mass flowmeter, in which two practically parallel bent tubes are fabricated from one bent tube as shown in FIG. 5 and thereby the fluid to be measured is caused to flow in series, is also known as an improvement on the above Coriolis mass flowmeter.
In tubes of this loop type, however, the problem resulting from the formation of the above branching and junction portions can be solved, but their connection to external piping is complex and difficult because of their three-dimensional (not two-dimensional) geometry. At the same time, measuring accuracy is affected by strains caused during the fabrication of flow tubes, and the tubes of the loop type are susceptible to the effect of external vibrations.
To solve the above problems, an object of the present invention is to provide a Coriolis mass flowmeter which has a simple connection to external piping and can be assembled in a simplified manner while maintaining the advantage of the Coriolis mass flowmeter having a parallel double bent tube, that is, freedom of pressure losses or clogging with a fluid by bending a single tube in the above-mentioned manner to eliminate branching and junction portions.
Another object of the present invention is to provide a Coriolis mass flowmeter which can measure mass flow rate with high accuracy by reducing strains caused during working and at the same time by reducing the vibrations of external piping, etc.
The Coriolis mass flowmeter according to the present invention is a type in which a fluid to be measured is caused to flow in flow tubes connected in series by forming two parallel flow tubes 1 and 2, which are obtained by bending a single tube. The Coriolis mass flowmeter is characterized in that the inlet portion of conduit between an external connecting portion 35 connected to inlet piping and an inlet of one flow tube 2 is bent in an L shape and further bent in another direction, and the outlet portion of conduit between an external connecting portion 36 connected to outlet piping and an outlet of the other flow tube 1 bent in an L shape symmetrically with respect to the inlet portion of conduit, with an additional bend in another direction combined, with the result that the external connecting portions 35 and 36 are arranged on the same axis. This facilitates horizontal external connection during assembly. As mentioned above, loop-type bent tubes need to be three-dimensionally bent. In the present invention, this three-dimensional bending can be accomplished with the connecting portions of the inlet portion of conduit, outlet portion of conduit and two flow tubes. Thus, flow tubes that require working without generation of strains can be formed by two-dimensional bending alone.
A fixing member 32 of hollow construction is provided to integrally fix each end of the two flow tubes 1 and 2 via a supporting member 33 fabricated from a separate sheet and there is also provided an outer casing 30 which further supports this fixing member 32. The supporting member 33 is fixedly fitted to the fixing member at two locations opposite to the hollow fixing member 32 for each end of each of the flow tubes 1 and 2 and, at the same time, fixes the ends of the flow tubes 1 and 2. This facilitates the installation and fixing of the flow tubes and can prevent unwanted torque, vibration and wobble of the flow tubes. In the present invention, firm supporting is achieved at low cost with the use of supporting members fabricated from sheets and a hollow fixing member for fixing the supporting members, thereby reducing the vibrations of external piping, etc. and permitting a high-accuracy measurement of mass flow rate.
The outer casing 30 has a hollow shell construction and the fixing member 32 is supported by the outer casing at a plurality of points (supporting portions 38) independently, not over the whole surface. Furthermore, a pressure-resistant case 31 is provided to be tightly connected to the outer casing 30. This pressure-resistant case 31 accommodates one conduit including the flow tubes and all accessory components of the conduit that include a drive 15, a pair of detecting sensors 16 and 17 and the fixing member 32. In the present invention, the outer casing has a shell construction and the pressure-resistant case is connected integrally to the outer casing. This not only enables the vibrating structural portions of flow tubes to be strongly protected against stresses from external piping, but also prevents the liquid flowing through the tubes from flowing out of the pressure-resistant vessel even in case of a breakage of the tubes.
Furthermore, the method of making the Coriolis mass flowmeter according to the present invention involves integrally assembling the conduit and accessory components of the conduit, such as the fixing member 32 for fixing each end of the flow tubes, the drive 15 and the pair of detecting sensors 16 and 17, as a unit. This unit is arranged in the outer casing 30 having a shell construction in a manner that the outer connecting portions 35 and 36 are on the same axis. This facilitates assembling and can reduce the assembling time.